The present invention relates generally to the field of test apparatus, and more particularly to the field of test apparatus for thin film coatings applied to the surface of articles.
Articles formed of plastic materials are often coated with a thin film coating in order to enhance certain properties of the resulting article. In particular, plastic articles are often used as containers for food, beverages, or the like, but such containers are generally found to be lacking in certain physical properties. Thin film coatings are therefore used to enhance the physical properties of the containers. One of the most common purposes for such coatings is to form a barrier against permeation of gasses through the container walls. For example, reduction of the oxygen permeation rate helps to retard spoilage of a foodstuff in the container, while reduction of the carbon dioxide permeation rate helps to maintain a high level of carbonation in carbonated beverages. One common coating which is widely used is an SiO.sub.2 coating, which forms a very thin, glass-like layer having very good barrier properties.
A major difficulty with volume production of such coated containers is that periodic sampling of production containers is required to ensure that the coating is performing to the desired specifications. In the prior art, the periodic sampling required actual, direct measurement of the physical property of interest. Some of the required measurements, however, are extremely painstaking and time consuming to perform. In the case of permeation rate, for example, the actual, direct measurement thereof requires a substantial amount of large test equipment and apparatus, including gas supply, plumbing, an environmental enclosure, precision gas detectors, etc. In addition, the permeation rates of the coated containers are so low that a test cycle usually lasts for several weeks, and typically spans eight weeks, before a steady state permeation condition can be achieved and accurately measured.
Accordingly, a need exists for a faster, simpler method for measuring the macroscopic physical properties of a thin film coating, e.g. those properties which are macroscopically observable and/or measurable. That goal is achieved in the present invention.